Display system

ABSTRACT

Apparatus for generating characters or symbols by deflecting the electron beam of a cathode ray tube with signals derived from a read only character memory. The character memory provides an output under the influence of memory control. This output is loaded into registers and then fed as a bit stream to a decoder. The decoder interprets the bit stream and sends X and Y increments and control signals to digital to analog converters which generate strokes to form the desired characters on the screen of the cathode ray tube.

United States Patent 1 1 INPUT SOURCE F la 511%.

ADDRES S SELECTOR Rosenthal Dec. 25, 1973 DISPLAY SYSTEM 3,325,8026/1967 Bacon 340/324 A [75] Inventor: Jerome A Rosemhal, Brighton,3,537,098 10/1970 Nielsen et al 340/324 A Primary Examiner-David L.Trafton [73] Assignee: Xerox Corporation, Stamford, Attorney-James J.Ralabate et al,

Conn. 22 Filed: Dec. 28, 1971 [57] ABSTRACT Apparatus for generatingcharacters or symbols by de- 1 A P 5 212,934 flecting the electron beamof a cathode ray tube with signals derived from a read only charactermemory. [52] US. Cl. 340/324 A, 315/18 The Character memory Provides anOutput Under 51 Int. Cl. G06f 3/14 influence of memory control ThisOutput is loaded [58] Field of Search 340/324 A into registers and fedas a bit Stream to a coder. The decoder interprets the bit stream andsends 5 References Cited X and Y increments and control signals todigital to UNITED STATES PATENTS analog converters which generatestrokes to form the desired characters on the screen of the cathode ray3,540,032 11/1970 Cr|sc1magna et al. 340/324 A tuba 3,696,388 10/1972Eichelberger 340/324 A 3,329,948 7/1967 Halsted 340/324 A 4 Claims, 2Drawing Figures ENABLE x DATA X T0 x M assets- Z-BlT 01200 3512 To YDEFLECTION STEER'NG AMP OF CRT LOGlC 2 -ro BLANKING 18 AM OF CRT END OFRESET J CHAR CKT. 1

DISPLAY SYSTEM This invention relates to character generating apparatus.More particularly, this invention relates to an electronic charactergenerator in which the electron beam of a cathode ray tube is deflectedin order to form characters or symbols on the tube screen.

BACKGROUND OF THE INVENTION With the advent of the computer and otherrapid data transmission devices, there has come a need for systems whichcan effectively and speedily display the intelligence from thesesources. Improvements in electromechanical printers have substantiallyincreased speed. However, because of the problems of mechanical inertia;electromechanical printers soon reach a practical upper limit speed. Tofill the need for fast responding display devices a number of electronicdevices have been proposed. Among these is the character generatorapparatus employing a cathode ray tube that may em ploy matrix, bitstream or'stroke character encoding. A comparison of matrix, bit streamand stroke character encoding would show that each has certainadvantages regarding such factors as memory size, ease of coding,character quality, exposure efficiency and circuit complexity. Matrixencoding systems have the disadvantages of poor exposure efficiencywhile requiring a large memory. However, matrix encoding has theadvantages of coding ease and simple circuit configurations withmoderate character quality. Stroke encoding systems have thedisadvantage of difficult coding and complex circuit configuration.Nevertheless, stroke encoding systems do have the advantages of goodcharacter quality and exposure efficiency with small memoryrequirements. Lastly, the advantages of bit stream character encodingsystems fall roughly between that of matrix and stroke systems in thatmemory size and circuit complexity are moderate whereas coding,character quality and exposure efficiency are good. This inventionincorporates the advantages of ease of coding and circuit simplicity ofthe matrix character encoding system as well as the advantages of goodcharacter quality and exposure efficiency of the stroke characterencoding system. In short, this invention employs a bit stream encodingsystem modified to incorporate the advantages of both the matrix andstroke character encoding systems.

Character generation by means ofa cathode ray tube hasthe objective ofdisplaying line traces on the face of the tube or screen. The linetraces may represent vectors, alphanumeric characters or symbols. Toprovide such a display, the electron beam of the cathode ray tube isdeflected from point to point along a predetermined path. To deflect theelectron beam along the predetermined path, it is necesssry at eachpoint of direction change in the path to apply the proper deflectionsignals to the horizontal (X-axis) and vertical (Y- axis) deflectioncircuits of the cathode ray tube to move the electron beam to the nextpoint of direction change. In digitally operated display systems, thesedeflection signals are stored in the character memory and applied to theinput circuits of the cathode ray tube at timed intervals. Digitallyoperated systems of the past required a great deal of programming andcalculations of values of deflection signals in order to drive theelectron beam across the tube screen. In addition, depending on thenumber and type of symbols to be generated,

digital storage and logic circuits were generally quite numerous. Alsothe deflection control circuits of these prior art display systems hadthe disadvantages of requiring precision components as well as beingdifficult to control.

In these prior art systems a multiplicity of current sources such astransistors and balancing resistors are utilized in order to generateline segments of characters. Since individual transistors inherentlyhave different operating characteristics, compensating circuitry isrequired to offset their differing characteristics. The effects of driftand the difficulty of maintaining equal character stroke intensity arealso disadvantages frequently encountered in these systems. Thesedeficiencies of prior art systems served not only to increase thecomplexity of the scanning control but also to increase the expense ofthe display apparatus.

Accordingly, it is an object of this invention to provide an improvedand inexpensive character generating apparatus.

It is another object of this invention to provide a character generatingapparatus requiring less current sources for generating characterstrokes.

It is a further object of this invention to provide a charactergenerating apparatus wherein all the character strokes are confined to agrid simplifying the coding of stroke signals.

It is a further object of this invention to provide a charactergenerating apparatus wherein the electron beam of the cathode ray tubeis automatically deflected to trace a symbol or character by a data bitstream input to circuits which control tube deflection.

SUMMARY OF THE INVENTION In accordance with principles illustrative ofthis invention, a cathode ray tube is advantageously provided forcharacter generation. Stroke generators activate the deflection circuitsof the cathode ray tube by delivering fixed voltage pulses to currentsources which in turn charge capacitors. The strokes are coded as binarybit streams for the X and Y axes. A binary "1" bit causes a fixed amountof charge to be deposited or removed from the capacitor associated withappropriate coordinates of X and Y. A binary 0" bit causes no change inthe quantity of charge. The entire bit stream for constructing thecharacter is stored in read only memory. The bit stream is read out ofmemory and stored in shift registers by memory control and then feed todecoder and steering logic. The decoder and steering logic interpretsthe bit streams and directs the X and Y incremental bit streams alongwith bits representing the signs of X and Y to the X and Y digital toanalog converters. The decoder and steering logic also generates thesignals for controlling beam blanking and end of character. The end ofcharacter signal is fed to a reset circuit which provides means forpositioning the characters in the character areas as well as resettingthe digital to analog converters in preparation for the generation of anew character. The blanking signal is applied directly to the blankingamplifier of the cathode ray tube. The outputs from the X and Y digitalto analog converters are applied directly to the X and Y deflectionamplifiers. The invention also provides means in the decoder andsteering logic for distributing the 0" and l bits within the stream sothat digitization error is minimized.

DESCRIPTION OF THE DRAWING The foregoing will be more readily understoodupon a reading of the following description in conjunction with thedrawing in which:

FIG. 1 is a block schematic diagram of an illustra-tive embodimentincorporating the principles of the present invention; and

FIG. 2 shows a sample character and its code.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1 there isdepicted a block schematic diagram of an illustrative embodimentincorporating the principles of this invention. All of the circuitry toimplement this embodiment is well known to one of ordinary skill in theart and no detailed description thereof will be given. Six binary bitsof information from an input source 1, such as a computer, datakeyboard, magnetic tape, etc., are fed into address selector 11. The sixbinary bits are character selection bits and may be in ASCII, EBCDIC orother standardized code. In address selector 11 the six bit binary codeis translated into a nine bit binary code designating the startingaddress for each character in character memory 12. This translation maybe done by any of various well known prior art techniques. Memorycontrol controls the transmission of the translated address to charactermemory 12. It is to be noted that any codes specified herein areintended to be illustrative and not limiting, since it will occur tothose skilled in the art that other codes could be used depending uponthe number of characters or symbols desired and the amount of storageavailable.

Character memory 12 illustratively is organized into 16-bit words. Thesewords are each subdivided into an eight-bit word for the X coordinateand an eight-bit word for the Y coordinate. Information for controllingthe generation of a character is stored as a block of contiguous words,the aforementioned translated address defining the first word of theblock.

Upon the selection of a character, a 16-bit word is provided at theoutput of character memory 12. The two eight-bit words within the 16-bitword contain the respective X and Y incremental pulse trains as well ascontrol bits such as blanking, unblanking, X and Y sign, and end ofcharacter. From character memory 12 one eight-bit word for the Xcoordinate is loaded in parallel into the X shift register 16 and oneeight-bit word for the Y coordinate is loaded in parallel into the Yshift register 17. Both the X and Y eight-bit words are stored in theirrespective registers 16 and 17 until directed by memory control 10,clock 13 and gate 14 to send a serial bit stream to two-bit decoder andsteering logic 18. Decoder and steering logic 18 interprets the bitstream and directs the X and Y incremental bit streams to X and Ydigital to analog converters 20 and 21, respectively. ln X and Y digitalto analog converters 20 and 21, the binary bits are integrated bycurrent sources which in turn charge or discharge capacitors and areconverted into electron beam strokes for the cathode ray tube havingdirection and amplitude.

The strokes are coded as binary bit streams for the X and Y axes. Abinary 1 bit causes a fixed amount of charge to be deposited or removedfrom the capacitor associated with the appropriate X and Y coordinates.A binary 0" bit causes no change in the quantity of charge on thecapacitor. Decoder and steering logic 18 also generates sign X, sign Y,blanking and end of character pulses. For example, a stroke which has anX increment of four and a Y increment of two requires a bit stream of:

l l l l 1001, the upper line being the bit stream for the X incrementand the lower line for the Y increment. For each stroke, the X and Y 1bits are placed in the stream to cause minimum digitization error. Thisis accomplished by arranging the bit stream so that it most accu ratelyapproximates the actual character shape as shown in FIG. 2 and discussedmore fully hereinafter.

In order to control the bit stream, it is necessary to insert controlinformation into the bit stream. When there is a change in X sign, Ysign, blanking, or to indicate end of character, control bits are sentalong in the bit stream. Provision is made to insert three bits ofcontrol information into the X bit stream and three bits of informationinto the Y bit stream. The first control bits in each stream are a codewhich signifies that the next two bits in the stream are controlinformation. In one implementation of the invention, the control code is0,0" this 0,0" combination not being permitted to occur in the X and Yincrement bit streams. The second two bits of the control informationindicate X sign and Y sign. The third two bits indicate blank, unblankor end of character. Some examples of control bits in thisimplementation are:

000 X positive, Y negative, beam unblanked,

000 X negative, Y negative, beam blanked,

001 end of character.

The stroke from X and Y digital to analog converters 20 and 21 areapplied directly to the X and Y deflection amplifiers of the cathode raytube (not shown). An end of character signal from decoder 18 is applieddirectly to reset circuit 22 which positions the character in sequenceon the cathode ray tube and also resets the capacitors in X and Ydigital to analog converters 20 and 21 in preparation for the start of anew character. Finally, a blanking signal from decoder 18 is applieddirectly to the blanking amplifier of the cathode ray tube (not shown).

A sample character and its code are shown in FIG. 2. In order to codethis character, using a character height of 24 units, I bits of memoryis required. Recalling that the top line represents an X increment bitstream and the line below it represents a Y increment bit stream, thecharacter A is formed on the grid as described hereinafter. For the sakeof illustration the bit streams have been blocked into frames numberingl-23. In frame 1 it is seen that it takes six bits to move the electronbeam to the starting point of the letter A. The first two 0,0 code bitsof frame 2 indicate that four control bits are to follow. The two I, 1bits indicate sign X positive and sign Y positive. The next l,0 bitsrepresent beam unblanked. Frames 3-8 initiate the strokes to form thefirst leg of the letter A moving upward. At the end of frame 8 a changeof direction is required. Therefore. frame 9 contains control bitsindicating X positive, Y negative and beam unblanked. Frames -15initiate the strokes to form the second leg of the letter A movingdownward. At the end of frame change of direction as well as blanking isrequired. Control bits in frame 16 provide for a negative X increment, apositive Y increment and beam blanked. Frames 17 and 18 provide bits tomove the blanked beam up to point P. At this time frame 19 containscontrol bits fora negative X increment, a negative Y increment, and beamunblanked. The crossbar is formed by the, bits in frames 20-22. Frame 23contains end of character bits which prepares the system for formationof a new character of symbol.

As can be seen from FIG. 2 the resultant character is generated incontinuous line segments of substantially equal stroke lengths on thegrid. This permits characters having substantially constant intensity.Moreover, the characters are formed at high speed and have goodcharacter definition independent of character size and location.

It will be apparent to those skilled in the art that the presentinvention may be used generally in display systems and is not limited tothe embodiment described. Numerous other variations, modifications andadaptations of the present invention will be apparent to those skilledin the art, and such as come within the spirit and scope of theappendedclaims are considered to be embraced by the present invention.

What is claimed is:

l.11Character generating apparatus for controlling the horizontal andvertical deflection circuits of a cathode ray tube to move an electronbeam across the screen of the cathode ray tube to display a characterthereon, said apparatus comprising memory means including addressablestorage locations for storing indicia representing characters, saidindicia including horizontal and vertical deflection incrementinformation,

means for receiving a character code defining a character to bedisplayed and translating said received code into a memory means addresscorresponding to the character defined by the received character code,

means responsive to said memory means address for retrieving from saidmemory means the indicia representing said defined character,

horizontal and vertical deflection increment shift register means,

means for loading the horizontal and vertical deflection incrementinformation of said retrieved indicia in parallel fashion into therespective shift register means,

means for serially shifting said horizontal and vertical deflectionincrement information of said retrieved indicia out of the respectiveshift register means as respective bit streams for the horizontal andvertical deflection circuits of said cathode ray tube,

a single horizontal converter and a single vertical converter forconverting the respective bit streams into stroke signals forapplication to the respective deflection circuits, and

logic means responsive to said bit streams for controlling thedeflection circuits of said cathode ray tube in accordance with thestroke signals to display said defined character on the screen of saidcathode ray tube.

2. The apparatus of claim 1 wherein said logic means includes means forrecognizing control signals in said bit streams indicating direction oftravel for said electron beam and blanking of said electron beam, andresponsive to said control signals for controlling said deflectioncircuits in accordance with said control signals.

3. The apparatus of claim 2 wherein said logic means further comprisesmeans for recognizing an end of character control signal in said bitstreams for resetting said deflection circuits so as to position saidelectron beam for the display of another character.

4. Character generating apparatus comprising a cathode ray tube having Xand Y axis deflection control circuits for controlling the movement ofthe electron beam of the cathode ray tube across the screen of saidcathode ray tube,

address selector means for receiving a character code from a data sourceand transforming said character code into an address code,

character memory means responsive to said address code for eachcharacter in memory for furnishing two words which contain control bitsand respective X and Y incremental pulse train bits,

shift register means for receiving said two words and for convertingeach of said words into a bit stream,

decoder and steering logic means for interpreting said control bits insaid bit streams,

a single X digital to analog converter and a single Y digital to analogconverter responsive to said decoder and steering logic means forconverting the respective X and Y incremental pulse train bits in saidbit streams into character stroke signals, and

X and Y deflection amplifiers responsive to said character strokesignals for deflecting the electron beam on the screen of said cathoderay tube to form characters thereon.

1. Character generating apparatus for controlling the horizontal andvertical deflection circuits of a cathode ray tube to move an electronbeam across the screen of the cathode ray tube to display a characterthereon, said apparatus comprising memory means including addressablestorage locations for storing indicia representing characters, saidindicia including horizontal and vertical deflection incrementinformation, means for receiving a character code defining a characterto be displayed and translating said received code into a memory meansaddress corresponding to the character defined by the received charactercode, means responsive to said memory means address for retrieving fromsaid memory means the indicia representing said defined character,horizontal and vertical deflection increment shift register means, meansfor loading the horizontal and vertical deflection increment informationof said retrieved indicia in parallel fashion into the respective shiftregister means, means for serially shifting said horizontal and verticaldeflection increment information of said retrieved indicia out of therespective shift register means as respective bit streams for thehorizontal and vertical deflection circuits of said cathode ray tube, asingle horizontal converter and a single vertical converter forconverting the respective bit streams into stroke signals forapplication to the respective deflection circuits, and logic meansresponsive to said bit streams for controlling the deflection circuitsof said cathode ray tube in accordance with the stroke signals todisplay said defined character on the screen of said cathode ray tube.2. The apparatus of claim 1 wherein said logic means includes means forrecognizing control signals in said bit streams indicating direction oftravel for said electron beam and blanking of said electron beam, andresponsive to said control signals for controlling said deflectioncircuits in accordance with said control signals.
 3. The apparatus ofclaim 2 wherein said logic means further comprises means for recognizingan end of character control signal in said bit streams for resettingsaid deflection circuits so as to position said electron beam for thedisplay of another character.
 4. Character generating apparatuscomprising a cathode ray tube having X and Y axis deflection controlcircuits for controlling the movement of the electron beam of thecathode ray tube across the screen of said cathode ray tube, addressselector means for receiving a character code from a data source andtransforming said character code into an address code, character memorymeans responsive to said address code for each character in memory forfurnishing two words which contain control bits and respective X and Yincremental pulse train bits, shift register means for receiving saidtwo words and for converting each of said words into a bit stream,decoder and steering logic means for interpreting said control bits insaid bit streams, a single X digital to analog converter and a single Ydigital to analog converter responsive to said decoder and steeringlogic means for converting the respective X and Y incremental pulsetrain bits in said bit streams into character stroke signals, and X andY deflection amplifiers responsive to said character stroke signals fordeflecting the electron beam on the screen of said cathode ray tube toform characters thereon.